Draw/relax/anneal process for polyesters

ABSTRACT

THE DYEABILITY OF CRYSTALLINE, LINEAR CONDENSATION POLYESTER FIBERS IS GREATLY IMPROVED BY SEQUENTIALLY DRAWING RELAXING AND ANNEALING THE FIBERS. THE FIBERS ARE DRAWN AT A TEMPERATURE ABOVE THEIR APPARENT MINIMUM CRYSTALLIZATION TEMPERATURE, ELAXED BOVE 180*C., AND ANNEALED AT A TEMPERATURE ABOVE THE RELAXATION TEMPERATURE. APPARATUS IS ALSO DISCLOSED.

' June 12, 1973 E. F. EVANS ET AL DRAW/RELAX/ANNEAL PROCESS FOR POLYESTERS 2 Sheets-Sheet 1 Filed Oct. 12. 1971 June 12, 1973 EVANS ET AL 3,739,056

DRAW/RELAX/ANNEAL PROCESS FOR POLYESTERS Filed Oct. 12, 1971 2 Sheets-Sheet 2 United States Patent US. Cl. 264-290 T 0 Claims ABSTRACT OF THE DISCLOSURE The dyeability of crystalline, linear condensation polyester fibers is greatly improved by sequentially drawing, relaxing and annealing the fibers. The fibers are drawn at a temperature above their apparent minimum crystallization temperature, relaxed above 180 C., and annealed at a temperature above the relaxation temperature. Apparatus is also disclosed.

CROSS REFERENCE TO RELATED APPLICATIONS This application claims benefit of the filing date of the parent application, Ser. No. 698,623, filed Ian. 17, 1968, by the same applicants, now abandoned.

BACKGROUND OF THE INVENTION Field of the invention This invention relates to an improved method for producing a highly dyeable polyester fiber and, particularly, a crystalline polyester fiber.

Description of the prior art Polyesters such as polyethylene terephthalate are relatively difiicult to dye in comparison with other synthetic polymers. The draw-relax-anneal process of the invention increases dyeability of polyester fibers with minimal loss of physical properties and at productive operating speeds by drawing the fibers above their apparent minimum crystallization temperature and subsequently relaxing and annealing them with the annealing temperature greater than the relaxation temperature, as hereinafter described. Improving the dyeability of polyester fibers is the subject of many prior disclosures, for example, US. Pat. Nos. 3,190,718 and 3,107,140 and British Pat. Nos. 1,050,393 and 1,012,461. None of the prior art, however, solves the polyester dyeability problems in the manner of the present invention.

SUMMARY OF THE INVENTION The invention is a process for improving the dyeability of linear, condensation polyester fibers comprising, sequentially, the steps of (a) drawing the fibers at a temperature above their apparent minimum crystallization temperature, preferably above 100 C., (b) relaxing the drawn, crystalline fibers at a temperature greater than the drawing temperature and at least about 180 C., and (c) stabilizing or annealing the fibers by heating them under tension at a temperature greater than the relaxation temperature and less than the fiber softening temperature to further crystallize the fibers. During stabilization the fibers cannot be stretched more than they were relaxed during relaxation. Preferably the stabilizing temperature is no greater than 235 C. and the relaxation temperature is at least 15 C. less than the stabilizing temperature.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic illustration of a continuous process and apparatus useful for practice of the invention.

Patented June 12, 1973 FIG. 2 is a sectional view of a steam jet device for use in the process, the cross-section being taken along the axis of the path of the fibers.

FIG. 3 is a schematic illustration of an apparatus suitable for use in relaxing or annealing drawn fibers.

DESCRIPTION OF PREFERRED EMBODIMENTS Referring to FIG. 1, amorphous polyester fibers 2 from a suitable supply are passed over roller guide 4 and enter the feed section of draw machine 6. The fibers pass around eight feed rolls 8-22 and are preheated between rolls 14 and 16 by bath 23. The fibers are drenched by a heated liquid sprayed from spray jets shown collectively at 24 to heat the fibers while they are being drawn. The fibers then pass around the eight draw rolls 26-40 and exit from the draw machine. The drawn fibers pass around roller guide 42 and nip roll 44 and enter steam jet 46. The drawn fibers are relaxed in the jet by feeding them faster than they are removed and by heating them with the impinging steam. The relaxed fibers then pass to annealing roll 48. The annealing oven 47 contains a series of heated appealing rolls, e.g., as many as 20 or more and which rotate at the same speed so as to heat the fibers at constant length. The annealed fibers are then treated with a cooling spray in cooler 50 and passed to puller rolls 52, 54 and 56. The fibers are then led to crimper 58, around guide roll 59 and through dryer 60 before being packaged.

FIG. 2 shows a suitable steam jet device for use as steam jet 46. For convenience in manufacture, the jet is made of three pieces which are bolted together for use. Body plates 64 are cut away to form treating chamber 71. Face plate 62 containing fiber passageway 70 completes the assembly. The joining of the three plates forms steam chambers 72 and steam passageways 74. The openings 68, which are suitably threaded for attaching a steamsupply line, conduct steam into the chambers 72.

FIG. 3 shows suitable apparatus for relaxing or annealing polyester fibers. Polyester fibers 76, that have been crystalline drawn in a separate operation, pass between feed roll 78 and nip roll 80. The fibers contact the metal surface of heater 84 aided by roller guides 82 and 86 for greater length of contact. Take-up roll and nip roll 88 rotate at a slower speed than the feed roll so that the fibers shrink a predetermined amount when heated to the relaxing temperature. The apparatus may be used to anneal the relaxed fibers or they may be passed to associated annealing means as part of a continuous relax-anneal process. When used as an annealing means the take-up roll preferably will operate at the speed of the feed roll and the heater will be maintained at a relatively higher temperature than is used for relaxing.

When polyester fibers are heated under tension at a temperature above the temperature at which they were relaxed, the fibers are said to be annealed. When polyester fibers are drawn, relaxed and annealed in conformity with this invention, the fibers show a surprising in crease in dyeability. Since it is known that to anneal oriented fibers at constant length produces no change in dyeability. or can even reduce this important property, the results of this invention are completely unexpected.

When a drawn polyester fiber is heated to a temperature above the highest temperature that the fiber has previously reached, at temperatures up to its softening point, the polyester fiber will exhibit a retractive force. In addition the polyester fiber will assume a crystallinity that is characteristic of the highest temperature the polymer reaches. If no restraint is imposed upon the fiber while it is being heated, the fiber will undergo a reduction in length corresponding to the magnitude of the retractive force. If the fiber is prevented from undergoing a change in length to an extent consistent with the retractive force,

the fiber will be under tension in proportion to whatever change in length is permitted. If the fiber is heated at consant length, the tension due to the retractive force will be at its maximum for the prevailing conditions. By permitting the fiber to yield to its retractive force, the orientation of the polymer molecules decreases and the fiber contracts. This change has been referred to variously as deorientation, relaxation and shrinkage; and while the terms are closely related, they are not synonymous. It is to be noted that when the term deorientation is used, reference is not made to an unoriented fiber but rather to one whose orientation has been reduced.

Fibers that have been permitted to fully yield to their retractive force at a given temperature and then cooled will not undergo any further change in length when they are subsequently heated to temperatures below the given temperature. Such fibers are said to be thermally stabilized. From the practical point of view, heating of the drawn fiber in a free-to-relax state results in an increase in dyeability and a decrease in tenacity and, of course, no dry shrinkage at temperatures below the relaxation temperature; heating the drawn fiber under tension results in structure stabilization and improvement in tenacity at the expense of dyeability. The fibers will exhibit a small amount, usually less than 5%, of dry shrinkage at or just below the treatment temperature. It is therefore highly unexpected that fibers can be treated to provide an increase in both dyeability and tenacity.

The dyeability of fibers produced by the method of this invention is dependent upon the degree to which they are relaxed and upon the annealing temperature. Sufiicient relaxation of crystalline drawn fibers to effect an appreciable change in dyeability is not achieved until a temperature of at least 180 C. is reached. The annealing temperature should be at least about C. above the relaxation temperature but below a temperature of 235 C. For some unknown reason, the tenacity of polyethylene terephthalate fibers decreases at an annealing temperature of 235 C. Therefore, it is preferred that the relaxation temperature be between 180 and 210 C. and the annealing tempearture between 195 and 225 C.

In the practice of the present invention, the fibers are drawn at a temperature above their apparent minimum crystallization temperature so as to achieve crystalline drawn fibers. Crystalline fibers have a higher stick point than amorphous fibers and thus can be relaxed at high temperatures with a minimum danger of fusing or sticking. The apparent minimum crystallization temperature is the lowest temperature at which an essential crystallization of the polyester takes place. As taught by the prior art, this temperature is at about 100 C. for most of the high-melting polyesters which are considered for the production of fibers. See US. Pat. No. 2,917,779. Since drawing is an exothermic process, fibers drawn in a bath maintained at a temperature of about 95 C. can achieve appreciable crystallinity indicating that the fiber temperature was at least about 100 C.

The expression apparent minimum crystallization temperature (T is defined as the lowest temperature at which a marked rate of density change, which is known to occur simultaneously with crystallization, takes place within six hours. A sample of polyester maintained at any given constant temperature below T; will not vary substantially in density over a long period, e.g., six hours. However, as soon as the polyester is subjected to the temperature T there occurs a rapid change in density. Actually, the rate changes quite abruptly from no change in six hours to a change within minutes for only a few degrees temperature difference. The value of T is conveniently assigned from density determinations done on fibers which have been heated in air or silicone oil and is based on crystallization by heat only.

The crystallinity in the fibers used in the practice of this invention can be measured using the crystallinity index method described by W. O. Statton in The Journal .4 of Applied Polymer Science" 7, 803 (1963). Fibers drawn in accordance with this invention will have a crystallinity index of at least 20.

The drawn yarns are obtained by drawing them at least twice their original length but less than their breaking elongation. Preferably, the fibers are drawn at a draw ratio of 2.5:1 to 5.0:1 and, more preferably from 3.0:1 to 5.021. The yarns are drawn while heated with a fluid maintained at the desired temperature. If desired, the yarns may be preheated prior to drawning, by passing them through a bath maintained at a temperature below, e.g., 20 C. below, the drawing temperature.

To relax the fibers they can be passed from feed rolls to a heating device and taken up by withdrawal rolls. The temperature of the fibers may be raised to the desired level by any suitable means such as contacting them with heated surfaces and heating with hot fluids. Preferably, heating is accomplished by impinging superheated steam onto the fibers. Alternatively, the fibers may be heated, as in a hot-air oven, in a free-to-relax condition. In the practice of this invention the fibers are relaxed from about 15% to about 30% and, more preferably 20% to 25%, of their drawn length.

During the relaxation step it is preferred that the fibers be under tension; that is, that the fibers be prevented from realizing the maximum relaxation attainable. The amount of shrinkage obtained is determined by the relative speeds of the feed rolls and withdrawal rolls and hence shrinkage can be accurately controlled. For instance, when it is desired to relax the fibers by permitting them to shrink 25% of their drawn lengths, the withdrawal rolls will be operated at a linear speed 25 less than the speed of the feed rolls.

The relaxation levels achieved by this invention are dependent on both the temperature of the heating means and the length of time the fibers are exposed to heat. The exposure time needed to provide economically use ful speeds will generally be on the order of 0.05-0.5 second. If desired, this value may be reduced somewhat by the use of high heating temperatures. The higher relaxation temperatures are required in order to obtain the higher relaxation levels during the most efficient exposure time. Since the amount of shrinkage that the fibers undergo is controlled, the fibers are relaxed under tension. The relaxation temperature must be high enough to keep the fibers under tension while being relaxed by the predetermined amount.

The fibers may be annealed by any suitable means, such as passing them around hot rolls or treating them with heated fluids. In a desirable embodiment of the invention, annealing rolls can serve as withdrawal rolls so that the fibers undergo a minimum heat loss in their travel from a relaxation treatment to an annealing treatment.

If desired, the fibers may be stretched or allowed to relax during the annealing treatment. The dyeability of the fiber is believed to be related to the net relaxation. The effect of stretching is to reduce the dyeability to a level below that which would otherwise be obtained and, hence, under most conditions stretching will be avoided. However, for circumstances where the dyeability loss can be tolerated, it may be desirable to anneal the fibers while they are undergoing a moderate amount of stretch, but such stretching should not exceed 5% of the relaxed length. As is known, stretching can be used to adjust fiber properties, e.g., fiber shrinkage. Shrinking of the fibers during the annnealing treatment also produces less than the optimum result. As the amount of relaxation allowed is increased twoards the maximum attainable, the results of this invention are reduced. Where some loss in the benefits of this invention is not too serious, shrinkage may be allowed to occur while the fibers are being annealed provide the change in length is small, i.e., less than 3% of the relaxed length.

During the annealing of the relaxed fibers, it has been found that the tenacity may also increase. Because of concepts that have come from the growth of the prior art knowledge, it ismost surprising to find that one can achieve an increase in both tenacity and dyeability in one step as practiced in this invention. From the knowledge that increasing the orientation of a fiber increased its tenacity and decreased its deyability has sprung the be-- lief that process steps that resulted in increased dyeability would lead to a decrease in tenacity. This belief is fortified by the hypothesis that in dyeing of polyesters only the noncrystalline regions are dyed, and therefore, a fiber having an increased dyeability would possess a noncrystalline region having increased deorientation and hence a reduced tenacity.

The measure of dyeability used herein is a dispersedye rate measurement. This disperse-dye rate is determined by dyeing the fiber in an aqueous dye bath having a temperature of about 100 C. for 9, 16 and 25 minutes. The dye bath contains 4%, by weight, based on the weight of the fiber, of a commercially available dye having the structure:

0 OH O-iil The weight ratio of bath to fiber is 1000:1. The amount of dye on the fiber is determined after the sample has been rinsed with water, then with acetone and dried in the oven. The dye is extracted with monochlorobenzene and determined quantitatively by measuring the absorbance at 449 my. on a spectrophotometer.

The disperse-dye rate (DDR) is obtained by dividing each of these three amounts of dye (percent by weight, based on fiber weight) by the square root of the dyeing time in minutes and averaging the values so obtained.

of the viscosity of a 10% solution of polyethylene terephthalate in a mixture of 10 parts of phenol and 7 parts of 2,4,6-trichlorophenol (by weight) to the viscosity of the phenol/trichlorophenol mixture, per se, measured in the same units at 25 C.

The following examples are intended to illustrate the practice of the invention and to contrast the invention with the prior art. Since the examples merely exemplify several embodiments of the process, no implication should be drawn that the invention is limited to the particular values of process parameters, e.g. draw ratio, which are exemplified.

EXAMPLE I This example illustrates the effect of the relaxation temperature on the dyeability of treated fibers.

Polyethylene terephthalate is spun into a 34-filament yarn, the polymer of which has a relative viscosity of 27.6. The yarn is drawn to a denier of 100 in a draw bath by feeding the yarn at 380 yards per minute and passing it to draw rolls rotating at a surface speed of 1530 yards per minute. The temperature of the draw bath is maintained at C. and the draw rolls at C. The drawn yarn has a crystallinity index of 20. This yarn serves as the supply for Samples A, B and C which are relaxed at C., C. and C., respectively.

Samples A through C are all relaxed using apparatus of the type shown in FIG. 3. The fibers are heated by contact with a hot shoe 75 centimeters in length. The fibers are forwarded to the heated shoe at 500 feet (153 meters) per minute and wound up at a slower speed to provide controlled maximum relaxation.

Using the the same apparatus used for relaxing, Samples A through C are then annealed at 205 C. at constant length at a speed of 300 feet (92 meters) per minute.

Results obtained by these treatments are given in Table I. From reference to the table it will be seen that there is a significant difference between the dyeability of the supply yarn and the dyeability of the yarn relaxed at 180 C. It is also seen that the dyeability of the annealed yarn is highest for the yarn relaxed at 180 C TABLE I.-CRYSTALLINE DRAW-RELAX-ANNEAL Relax Anneal Boil-01f Percent temp. temp. shrink- Sample overfeed 0.) C.) DDR T/E age Density Supply 0. 34 3 e 0 4. /33 13 3 1.369 15.1 3.7/48 2.0 1.381 18. 3 3. 6/51 1. 6 1. 384 21.6 3.5/60 1.1 1.388

The disperse-dye rates of 3-denier-per-filament fibers of EXAMPLE II this invention are preferably greater than 0.060. By comparison, the disperse-dye rate of conventionally prepared polyethylene terephthalate fibers having a denier per filament of 3 is 0.032.

As used herein, the terms percent relaxation and percent overfeed are considered to be equivalent. Percent overfeed is calculated from the formula:

Feed Speed-Removal Speed Feed Speed Percent Overfeed= X 100.

This example illustrates the effect of the annealing temperature on the dyeability of relaxed fibers.

The supply yarn of Example 'I is used to prepare samples which are relaxed at C., 205 C. and 220 C. These samples are all relaxed using apparatus of the type shown in FIG. 3. The fibers are heated by contact with a hot shoe 75 centimeters in length. The fibers are forwarded to the heated shoe at 500 feet per minute and wound up at a slower speed to provide controlled relaxatron.

Using the same apparatus used for relaxing, the relaxed samples are then heated at constant length at a speed of 300 feet per minute at various temperatures.

Results obtained by these treatments are given in Table II. From the table it is seen that when the relaxed fiber is heated above the relaxation temperature there is an improvement in the dyeability of the fiber and that this improvement increases with the annealing temperature. It is also seen that heating the fibers at constant length at a temperature below the relaxation temperature decreases the dyeability. Further, it is seen, and this is surprising, that at moderate increases in the annealing temperature, e.g., l530 C., and at temperatures below 235 C. both the dyeability and the tenacity increase while at a temperature of 235 C. dyeability increases and tenacity decreases.

TABLE II Relax Anneal Percent temp. temp. Sample overleed 0.) C.) DDR T/E EXAMPLE III This item illustrates the deleterious eifect on dyeability obtained by annealing crystalline-drawn fibers without a relaxation step.

A tow composed of 4,200 polyethylene terephthalate filaments of 8.5 denier per filament are drawn 3.2x at 95 C. The drawn fibers have a tenacity of 3.7 grams per denier, an elongation of 26% and a disperse-dye rate of 0.055. The drawn fibers are then annealed at 205 C. on hot rolls with a residence time of 7.2 seconds. The annealed fibers have a tenacity of 4.7 grams per denier, an elongation of 27% and a disperse dye rate of 0.028.

What is claimed is:

1. A process for improving the dyeability of linear, condensation polyester fibers comprising, sequentially:

(a) drawing the said fibers from 2 to 5 times their original length at a temperature above their apparent minimum crystallization temperature,

(b) relaxing the drawn, crystalline fibers from about to about 30 percent of their drawn length at a temperature greater than the drawing temperature and at least above 180 C., and

(c) stabilizing the said fibers by heating them under tension at a temperature greater than the relaxation temperature and less than the fiber softening temperature and at least 195 C. to further crystallize said fibers, and when said fibers are stretched or allowed to shrink during stabilization, said fibers being stretched no more during stabilization than they were previously relaxed and being allowed to shrink no more than 3 percent of their relaxed length during stabilization.

2.. The process of claim 1 wherein the relaxing temperature is at least 15 C. less than the stabilizing temperature and wherein the stabilizing temperature is no greater than 235 C.

3. The process of claim 2 wherein the fiber temperature during drawing is at least C.

4. The process of claim 2 -wherein the fiber temperature during relaxation is in the range of C. to 210 C.

5. The process of claim 2 wherein the fibers are drawn at a draw ratio of 2.521 to 5.021.

6. The process of claim 2 wherein the fibers are relaxed from 20 to 25 percent of their drawn length.

7. The process of claim 2 wherein the fibers are stabilized at constant length.

8. The process of claim 2 wherein the fibers are stretched during stabilization no more than 5 percent of their relaxed length.

9. The process of claim 2 wherein the fiber temperature during drawing is at least 100 C., the relaxation temperature is in the range of 180 C. to 210 C., and the stabilization temperature is in the range of C. to 225 C.

10. The process of claim 2 wherein the said polyester fibers are prepared from a polymer selected from the group consisting of polyethylene terephthalate, polytrimethylene terephthalate, poly'(pvhexahydroxylene terephthalate), poly(diphenylolpropane isophthalate) and the polyethylene naphthalene dicarboxylates.

References Cited UNITED STATES PATENTS 3,527,862 9/-1970 Shimosako et a1. 264290 T 3,562,382 2/1971 Fowler 264-342 RE FOREIGN PATENTS 735,171 8/1955 Great Britain 264-342 R 1,012,461 12/1965 Great Britain 264-342 R LORENZO B. HAYES, Primary Examiner J. B. LOWE, Assistant Examiner US. Cl. X.R.

264-4142 RE, 346, DIG. 73 

